Catamaran sailboats have developed over the past 60 years to now be a well-engineered and accepted choice of vessel for a large section of the public. Their increased use in the fields of public transport, tourism operations and private use, along with many design and construction improvements has led to a change in the attitudes of the maritime community to catamaran vessels. They are now well accepted by many in the maritime community.
One large factor still providing resistance to the greater acceptance of catamaran vessels is their great width. Catamarans have a beam measurement that is typically 60- to 70 percent of their length in width so many cruising catamarans may be as much as 24 feet wide. In many localities the extra area taken up by the greater width of a catamaran produces problems in their storage. Many marinas charge extra fees to berth a catamaran and some marinas are unable to accept catamarans due to the position of mooring poles and such. When catamarans need to be taken out of the water many slipways and travel lifts are unable to accept them due to the catamaran's great width. Catamarans are usually light and easily moved but their great width makes them difficult to remove from the water on normal equipment.
Catamarans have much potential as trailerable boats. Their light weight allows them to be towed behind a larger range of cars than ballasted boats. Their divided accommodations provide for the individual spaces necessary for a family cruiser. Their large cockpit along with the low level of heel makes using a catamaran an easy and fun experience. The problem until now has been how to reduce the catamaran's beam when required.
Many attempts have been made to overcome the problems of beam reduction and many patents issued on the topic. None of these has resulted in the trailerable catamaran becoming common due to a deficiency in the usefulness of each separate concept.
Many inventors have tried to modify the typical structures found on catamarans. Derek Kelsall uses telescopic beams in his approach in his Xcat designs. There is much that is correct with this approach in that the catamaran can be folded on the water and that the hulls do not rotate as extension or retraction occurs. The problems with this approach are that the mast cannot be in the raised position when undergoing extension or retraction and that large sleeves in the interior of the hulls compromise the amenity of the hull's interior. Extension and retraction is dependent on the beams sliding well inside their outer telescopic members and sleeves. This is very hard to achieve in practice as dirt and scratches increase the friction of the sliding parts. On top of this a small gap must be left between each telescopic section which increases the slack in the beam. In practice it has been found to be very hard to reduce beam with this arrangement and most catamarans of this type are rarely retracted once launched.
Builders using this design have reported much hardship in getting the sliding beams to slide during expansion and retraction. One of the great problems that the designer has been unable to rectify is how to produce a well engineered and easy to make sliding beam system. The many efforts of builders have gone into solving this problem but even with many minds working on the problem these boats have not been seen expanding and retracting as theory says they should.
One problem with the theory is scaling up the loads that occur in models. Although the telescopic approach can be made to work on small boats the physical fact that loads increase much faster than size, (the so called square/cube rule in which things get weaker as they get bigger, witness a small boat being able to easily lie on a rough surface whereas a vast ship would break its back) means that many theories that work well in small sizes do not work when applied to catamarans over model size.
Indeed one of the loads that had to be coped with during the development of the full size prototype on which this patent is based is the tendency of catamaran hulls to rotate, deck inwards, as the folding process occurs. Development was needed to overcome this in the prototype and the beams used in the prototype also had to move easily under heavy loads. To this end the telescopic beam used for carrying the mast and deck loads was ruled out and pivoting panel sets were used
Another problem with this telescopic beam approach is its structural integrity. Telescopic beams are not especially strong as they rely on each part being “buried” in its neighbour to pass along the forces they are designed to resist. If the amount of bury is increased the beam of the catamaran must be reduced which is undesirable, or the number of telescopic elements has to be increased which increases the complexity, weight and slack in the beam. A catamaran puts massive forces on its beams and this method is destined to be used in only a reduced number of small catamarans.
Another approach to the folding problem is to use a box-like framework which is disassembled when beam is reduced. This approach can produce a more structurally sound catamaran than the telescopic beam catamaran but has many problems. One of the main problems with this approach is that reducing the beam of the catamaran is a long and laborious process and cannot be done in a reasonable timeframe. On top of this bolt together assemblies are not highly suitable for the cyclic loads found in catamaran structures.
To get around the problems with the above methods catamaran designer Richard Woods has designed the “Wizard”. This catamaran has a central pod that contains the accommodations, crew cockpit and rigging attachments. The two hulls fold down underneath the pod to fit on a trailer. It has the benefit of being structurally sound, with its use of well proven hinges and deep beam bury, and of being quick to fold. Its major drawback is that it requires a special trailer and cannot be folded in the water. Additionally the hulls are not useful as storage or accommodation on the trailer as the hulls are rotated through 90 degrees when folded. The fact that it does work well is testament to the concept of using rotating structural members rather than sliding beams. However like Kelsall's catamarans the limitations placed on use, a special trailer is needed for folding and deep launching ramps needed for trailering, means that these boats are rarely built.
This method of using rotating structural members is the basis for the most successful folding multihull—a trimaran. Farrier (U.S. Pat. No. 3,937,166) uses hinges and crossbeams to rotate the outer hulls of a trimaran in a very strong and effective way. His method allows users to fold or retract the hulls underway and without special trailer arrangements. Best of all it is a very quick and easy method to use.
Farrier's method has proven very successful and over 2000 of his designs have been built around the world. However his method is only suitable for trimarans and not for catamarans as catamarans have very different structural requirements and loading conditions. Trimarans do not have the same advantages as catamarans in terms of accommodation and interior and exterior utility. Trimarans are rarely built as cruising boats these days due to these limitations.
Farrier's trimarans are also structurally robust. They highlight the efficacy of using rotating members such as struts to achieve a strong folding system. Of all the folding systems patented for reducing beam in a multihulled boat Farrier's is by far the best but its many advantages only apply to the trimaran configuration.
The use of rotating members in catamarans is not new. Many unpatented designs have used rotating panels such as one from the board of Roger Simpson. However the rotating panels proved unable to properly cope with the loads found on a catamaran out on the water and these boats were rarely used and often found with their beams bolted permanently to guard against structural failure. The vertically hinged panels were easy to build and could rotate easily without load but under loading they showed many signs of strain.
The use of rotating panels with hinges for catamarans was patented by many including Pelly (U.S. Pat. No. 5,522,339). This method uses thick flat panels with over centre clips as the structural members. The thick flat panels are aligned in the vertical plane to cope with the mast compression loads and other structural loads and have a hinge at the hulls inside edge and another at the centreline of the boat where the panels join onto their complimentary member. Flat panels are also used to from the cockpit floor. While this is a reasonable technique in some ways it does not cater for the extreme loads found in a catamaran's beam.
One difficult problem is that of keeping the two panels that make up the mast beam straight. The underside of the beam is under tension and this will stay straight naturally as the mast compression load is applied. The top of the beam however is under extreme compression forces, when mast loading is applied, and putting a hinge in a beam and then trying to keep it straight under compression is very difficult. To remedy this Pelly uses large pivoting compartments that take up valuable interior space when folded.
Putting hinges in the middle of a simply supported beam reduces the ability of that beam to handle load. The highest loads in a simply supported beam are found in the middle of the beam. Putting hinges in this area will not produce a robust structure.
Furthermore the mast compression loading is only one of the loads found on a catamaran. Many of the twisting and torque loads found when in rough water put huge forces on any crossbeam arrangement. Normal catamaran beams are not thin sandwich structures capable of taking loads only in the vertical plane but are box-beams capable of handling torsional loads and horizontal plane loads as well. A catamaran undergoes severe torsional loading when a gust hits the sails. The sidestays that hold up the mast are usually led back behind the centre of gravity of the hulls. This creates a torque as the hull is lifted so that the bow of the lifted hull drops compared to the stern of the same hull. Some large catamarans with low torsional stiffness resort to running anti-twist lines from the mast and cleated up forward (an example of this is an aluminum beam catamaran such as Tennant's Bladerunner). This loads the beam and mast more highly and is not a reliable method for beginner and social boat users.
Pelly's patent uses rotating members that have their axis of rotation in the vertical plane. This is typical of the vast majority of catamaran designs that use rotating panels to cope with the mast loading. The use of vertically rotating panels is useful for some areas of a catamarans structure but what Pelly and others including Francke (see below) do not recognise are the huge forces that try to rotate the hulls of a catamaran so that both hulls want to rotate their decks inwards as the boat is retracted. The loads on the folding mechanism are huge at point when the hinge and panel system becomes weaker due to their being in an angled orientation rather than in a straight orientation as when sailing.
What is needed is a way of coping with the twisting loads that a folding system will be unable to properly cope with during the retraction and expansion process. To this end a simple sliding tube or tubes arranged to keep the hulls in the vertical plane will stress the folding mechanism much less during retraction and expansion. As a benefit the tube or tubes can be arranged to provide a method of expansion.
U.S. Pat. No. 6,546,885 (Francke) discloses a variable width catamaran having tandem pairs of scissor-like folding mechanisms located between the hulls. Francke's patent is designed to allow retraction to occur with the mast still standing. Francke's patent uses very high quality and expensive components to cater for the highly stressed simply supported beams and connective structures. Additionally the mechanism intrudes into the accommodation along a considerable length of the hulls.
Francke's patent has problems that are shown in the changes to his prototype after its initial testing. Much credit must go to Francke for developing his prototype to full size but at full size complications due to unrecognized loads can prove detrimental. Although it had been folded with the mast on the centreline in a quiet waterway Francke's boat was quickly modified to a twin mast configuration that located each mast in each hull. As Francke's scissor like arrangement folds, the mast moves forward relative to the ends of the beams. This stresses the hinges and is probably one of the reasons that Francke has removed the single mast mounted on the centreline and put one mast in each hull. Obviously there are problems with a vertically rotating mast support system as the mast loading puts greater and greater side loads on the hinges in the structure as the catamaran folds.
A system where the mast supporting system consists of panels that rotated so that the mast was not displaced horizontally produces no extra strain on the hinges as the mast support panel set rotates. Panels could move about an axis that is substantially horizontal and substantially parallel to the centreline of each hull.
By using panels that rotate about a horizontal axis the mast will move in a vertical direction when expansion or retraction occurs. This will allow the mast step to be raised up when folded so that the mast step can easily be reached by sailors when rigging the boat. It also allows a higher deck to be designed so that interior volume is increased inside the hulls.
Another improvement is that the hinge point for a horizontally pivoting mast panel set needs to be straight and so the insides of each hull have to be straight along the hinges length. This increases the interior room of each hull and makes each hull easier to build from easy to obtain sheet material like plywood or foam sandwich.
The main problem with a horizontally pivoting mast panel arrangement is providing strength to resist the huge mast compression forces when each panel arrangement needs three hinge points to be able to fold up during the hull's retraction. The panels should be as thin as possible to allow the hulls to be as wide as possible. Trailering restrictions around the world restrict the beam of a catamaran on a trailer to widths from 8 foot to 8 foot 6 inches. This restriction produces a compulsion to reduce the panel thickness as much as possible to allow the greatest interior room in the restricted trailering width.
If the panels are to be used as simply supported beams like those of Pelly and Francke then thin panels would not be able to be used and a major structure would be needed to brace the centreline hinge from rotating under the mast loading. Obviously thin horizontally rotating panels cannot be used in the typical catamaran method of the simply supported beam. Almost all catamarans use the principle of the simply supported beam to support the mast between the hulls, this is normal catamaran practice. Most large catamarans use a very large simply supported beam, often incorporating the accommodations within in. In small versions where the simply supported beam is an aluminium tube it can be supported by a “dolphin striker”, a compression strut and wire arrangement that makes the beam into part of a truss system. The dolphin striker arrangement is commonly used on catamarans up to 18 meters long with tubular beams. These catamarans are usually racing orientated boats and do not have a folding capability. For transport, racing catamarans with aluminium or other beams are usually disassembled. This process can take a full day for a team of sailors when assembling a 40 ft racing catamaran.
Two multihull boat designers, Robin Chamberlin and John Hitch have used a pyramid structure to cope with the mast loads found on catamarans. Chamberlin designed and built “Excess” and Hitch designed and built “Wired”. On these examples the mast is stepped on a number of long compression struts, usually 3 a side, that are angled upwards from the hull inner side at angles of approximately 20 degrees and meet under the mast step. The mast compression loading induces a compression force in the tubes that tries to push the hulls apart but this is resisted by stout wires or beams that lock the structure into that of a stressed truss. This arrangement has proven much stiffer than the typical simply supported beam and dolphin striker arrangement and “Excess” was the first catamaran to sail deep into Antarctic waters, proving its structural integrity. This arrangement has many pitfalls. It is very wasteful of deck space, makes crew movement more difficult and is not at all suitable for folding. The compression struts take up interior room in the hulls as the need to be strongly fixed using large bulkheads and frames.
The approach of resolving the major mast loads into compression struts has been proven by the long voyages and structural robustness of the two boats that pioneered the concept. The challenge was to incorporate the lessons from these two boats and make them applicable to a folding mechanism.
When looked at as a whole a major problem with the folding catamaran patents listed above is that they fail to cope adequately with the huge cyclic loadings catamarans can generate whilst being easy and inexpensive to manufacture and promote successful folding and extension. Catamaran beams can rapidly change their loading conditions and in heavy seas the structure can have very high load reversals in a very short period of time. The success of Farrier's patent is due in part to his understanding that loading the connections under one type of load, either tension or compression results in a more stable and effective hull connection for multihulls. His mechanism is structurally sound and easy to use. The structure used in “Excess” and “Wired” is structurally sound and also uses only compression and tension members but is in no way foldable.
By using compression and tension members instead of a simply supported beam for the mast loading a catamaran can be much stiffer than a typical non folding three beamed catamaran. As the mast loading stresses the compression struts (or panels) and the tension members, the whole mast or deck supporting structure becomes locked into position. By making the mast support panels of a reasonable size other load conditions can also be easily dealt with. The hulls are kept parallel to each other, the hulls are kept at the same width and the hull's bows and sterns remain in line with each other. However the tendency of all catamarans to rotate each hull so that the deck rotates inwards, the displacement of each hull vertically and the tendency of a catamaran to rotate one hull relative to the other when lifting a hull is not resisted well.
In the previous uses of the compression strut in catamarans, found in Chamberlin's and Hitch's designs, the struts were buried deep into each hull at various points to resist these other loads. Obviously other ways of resisting these loads have to be found.
By using pivotally interconnected panels which are set in the vertical plane these other load conditions may be coped with properly. Other methods involving sliding tubes and other structures could also be used.
The following mechanism is able to cope with the loads imposed by a catamaran structure with a large safety margin, whilst being easy to use and increasing the catamaran's amenity.